Research

Why conduct research and distribute oral rabies vaccine too
Permission from the State to distribute oral rabies vaccine is contingent
on Fairfax County conducting research that adds to the body of knowledge
regarding oral rabies vaccine use. Although the safety and effectiveness
of the oral rabies vaccine have been established, vaccine distribution
strategies are still a matter of research.

What is being learned through the Fairfax County Oral Rabies Vaccine
Pilot Program?

Under the direction of Dr. Francois C. Elvinger, program manager of the
Fairfax County Oral Rabies Vaccine Pilot Program, the following research
has emerged:

Lori Thiele, pilot program coordinator, conducted a major study in
spring 2001 to evaluate the effect of 'fragmented' distribution of oral
rabies vaccine within the target area. Oral rabies vaccine distribution
in urban and suburban environments is constrained by factors that are
different from those encountered in rural areas where most oral rabies
vaccine is distributed. These include greater potential for human
exposure to the vaccine, greater competition for the vaccine bait by
companion animals, and property access and land use restrictions that
limit vaccine bait placement. As a result, uniform vaccine distribution
density even across uniform raccoon activity zones may not be feasible.
It is hoped that the immunity islands created within the target area by
a 'fragmented' distribution scheme will lead to satisfactory
vaccination of raccoons. To examine this, a group of contiguous
suburban parks with a combined surface area of 1,000 acres was
delineated into eight sections using squares formed by the County tax
grid. Alternate tax squares totaling 441 acres received oral rabies
vaccine at the rate of 1 dose per acre and the other four (a total of
559 acres) were not vaccinated. Of the one hundred vaccine doses
monitored across the four vaccinated squares, 87 doses were contacted
within four to five days of placement. Of the 100 raccoons then
live-trapped starting 24 days after vaccine distribution, 13 had blood
titers demonstrating prior exposure to the rabies antigen (titers
ranged from 1:25 to 1:1400), with eight positive raccoons trapped in
the four vaccinated squares and the remaining five trapped in the
unvaccinated squares. The averages of the positive titers were not
significantly different (P=0.88) when vaccinated and non-vaccinated
areas were compared, suggesting that raccoons exposed to oral rabies
vaccine move beyond the area in which oral rabies vaccine is
distributed. See linked map of
the 2001 test results(JPG file, 109K). Two interesting
additional findings came from the study:

Based on trapping 144 raccoons (44 prior to vaccine distribution
and 100 following with a combined 28% trap success), the raccoon
population on the 796 acres of parkland considered in the estimate
is 221 or about one raccoon per three acres (0.28 raccoons per
acre).

Of the 44 raccoons live-trapped and bled in six of the eight
squares prior to vaccine distribution, six had titers demonstrating
prior exposure to the rabies antigen (titers ranged from 1:13 to
1:110). This was a larger number of raccoons than expected and a
finding that deserves further study.

Although it is known that the spread of rabies in Anne Arundel and
Pinellas Counties, two suburban and urban counties in Maryland and
Florida, was effectively halted through oral rabies vaccination, the
proportion of raccoons that need to become vaccinated for a program to
be effective is still a matter of conjecture. Information from these
counties suggests that only less than half of the raccoons need to be
vaccinated for the disease to be effectively stopped. In the Fairfax
County pilot program, 37 percent of 89 blood samples and 45 percent of
66 blood samples collected after the first and second vaccine
distributions indicated exposure to oral rabies vaccine. The course of
rabies in a raccoon population that is vaccinated with oral rabies
vaccine can be modeled to establish an effective herd immunity level
for rabies to allow a better calculation of the resources needed to
organize an effective program.

There is a lot of competition by wildlife and companion animals for the
bait that contains the oral rabies vaccine. Opossums and foxes, and in
urban areas especially cats and dogs like to eat the bait, and thus
deprive raccoons of the vaccines. A study conducted by Jaime Landolfi,
a veterinary student in her summer internship program, attempted to
quantify competition by putting out hair sampler devices (hair salons)
to identify animals that picked up bait at central point feeding
stations, i.e. at those locations from which County citizens reported
raccoon activity (i.e., or involuntary feeding). Raccoons at those
locations took up most of the vaccines, however, an important
competitor for bait were cats.

Karen Wolf, a veterinary student and summer intern, evaluated wildlife
movement and competition for bait in culverts that are built under
highways like the Fairfax County Parkway and the Interstate that pass
through the county. She evaluated the potential for highways to
function as barriers to wildlife movement and consequently also to the
spread of rabies. She documented that there was a lot of traffic by
raccoons and other wildlife through culverts and that not only raccoons
but other wildlife species ate baits when in these culverts. This could
be expected to spread the immunity against rabies on the outskirts of
the target area, but also shows the potential for rabid animals to
migrate back into the target area.

Elizabeth Embree Schmidt, a junior veterinary and graduate student, for
her thesis project is studying the submissions for rabies testing and
incidence of rabies throughout Fairfax County. From 1995 through 1999,
the County laboratory tested 234 to 465 animals per year. This
constitutes a significant use of Health Department resources, and the
study focuses on spatial and temporal submission patterns and the
relationships between tested animal species and positive tests. The
outcome of her study may allow better prediction of where and when
cases may be detected at the laboratory and help in the planning of
County resources in the control of public exposure to rabies.